1. Field of the Invention
The present invention relates to an object distance measuring system for digitally displaying the distance between the camera and photographing object when an automatic focusing operation is carried out.
2. Description of the Prior Art
So far methods and apparatus for detecting the sharpness of the object image suited for optical instruments such as a camera have been disclosed, for example, in the Japanese Patent Publication Nos. Sho 39-29120 and Sho 41-14500 applied for patent of the applicant of the present invention and the U.S. Pat. No. 3,562,785 applied for patent by Dwin Richardson Craig (corresponding to DAS 1173327 and Japanese Patent Publication No. Sho 44-9501).
The Japanese Patent Publication No. Sho 39-29120 relates to a method for detecting the sharpness of the image object by knowing the variation of the internal resistance and that of the electromotive force of the photoelectric means taking place when an image of the object is formed on the photoelectric means by making use of the non-linear resistance-illumination characteristics considerably peculiar to the photoelectric means such as CdS or CdSe.
Further the Japanese Patent Publication No. Sho 41-14500 is an improvement of the instrument according to the Japanese Patent Publication No. Sho 39-29120, relating to an apparatus characterized in that two photoelectric means presenting a photoconductive material such as CdS are disposed before and behind the expected focus plane, whereby both photoelectric means are connected with a differential circuit in such a manner that the difference of the outputs between both photoelectric means due to the focus adjusting situation of the optics is detected so as to detect the focus of the optics.
Further, the U.S. Pat. No. 3,562,785 makes use of the principle of the Japanese Patent Publication No. Sho 39-29120, proposing a method characterized in that the image of an object is projected by means of optics on the image receiving plane of the two photoelectric means presenting a non-linear illumination-resistance characteristics in such a manner that the one photoelectric means receives a clear image and the other photoelectric means receives a dispersed image, whereby the optics is adjusted until the electrical response signal by means of both photoelectrical means reached maximum, by detecting the variation of the resistance value of the photoelectric means caused by the variation of the distribution of the light of the image receiving plane on the photoelectric means due to the variation of the sharpness of the image and an equipment for automatically adjusting the optics making use of the electrical response signal by means of both photoelectrical response signal.
The above mentioned non-linear resistance-illumination characteristics of the photoelectric means signifies a phenomenon such that as the sharpness of the object image formed on the photoelectrical means is increased, the electrical characteristics, especially the resistance value of the above mentioned photoelectrical means is increased or decreased, whereby the phenomenon is based upon the fact that the quantity of the incident light beam per unit area of the photoelectric material varies according to the variation of the sharpness of the object image in such a manner that at the maximum sharpness a considerable inclination takes place in the distribution of the quantity of the incident light beam on each point of the photoelectrical material.
In other words, the above mentioned phenomenon is based upon the fact that the difference of the quantities of the light beam among the bright parts and the dark parts of the object image is maximum when the sharpness of the object image is maximum in such a manner that the difference of the resistance values among each parts of the photoconductive material becomes large. However, as to the objects which are in real existence and often photographed, the distribution of the bright parts and of the dark parts is remarkably irregular whereby it can not always be said that the difference of brightness between the bright parts and the dark parts, namely the contrast is large, so that it must be said that only by forming an object image on the surface of a photoelectric means presenting a certain extent, consisting the above mentioned photoconductive material, a sufficient photoelectric effect can not be obtained and therefore it is difficult to obtain a very precise detection of the sharpness of the object image.